The Transitional Pre--Main-Sequence Object DI Tauri: Evidence for a Substellar Companion and Rapid Disk Evolution

We report mid-IR observations of two young stars found in the Taurus dark cloud, spatially resolving for the first time their 10 μm emission. The weak-emission T Tauri star DI Tau, tentatively identified by Skrutskie et al. on the basis of 12 μm IRAS data as an object in the process of dissipating its circumstellar disk, is found to have no infrared excess at a wavelength of 10 μm. The nearby classical T Tauri star DH Tau exhibits excess emission at 10 μm consistent with predictions based on circumstellar disk models. While both objects appear to have the same stellar mass, age, and rotation rate, they differ in two fundamental respects: DH Tau is a single star with an active accretion disk, and DI Tau is a binary system lacking such a disk. The companion to DI Tau has a very low luminosity and is located at a projected distance of ~20 AU from the primary. Assuming the system to be coeval, we derive a mass below the hydrogen burning limit for the companion. We speculate that the formation of a substellar mass companion has led to the rapid evolution of the circumstellar disk that may have surrounded DI Tau.